1. Field of the Invention
The invention relates to a controller and a control method for a vehicle in which a torque converter equipped with a lockup clutch is installed between a drive power source and a transmission.
2. Description of the Related Art
A technology is available by which when a lockup clutch that can directly couple an input side and an output side of a torque converter is controlled, a control region is set that provides a microslip to the lockup clutch, whereby the lockup actuation range is expanded and fuel consumption performance is improved. Such a control is called a flex lockup control, a slip lockup control, or simply a flex control or a slip control. A technique for conducting such a slip control of a lockup clutch when a vehicle starts moving is disclosed for example in Japanese Patent Application Publication No. 2005-3193 (JP-A-2005-3193).
A controller for a lockup clutch for a vehicle disclosed in JP-A-2005-3193 determines a target engine revolution speed for obtaining an engine output torque corresponding to the actual accelerator depression amount or throttle opening degree, and calculates a target slip amount (=target engine revolution speed−actual turbine revolution speed) for obtaining the target engine revolution speed. The controller also controls a torque capacity (transmission torque) of the lockup clutch so that the actual slip amount (=actual engine revolution speed−actual turbine revolution speed) of the lockup clutch is caused to follow the aforementioned target slip amount. Thus, where the actual slip amount is less than the target slip amount, the torque capacity (transmission torque) of the lockup clutch is reduced by a predetermined value and the actual slip amount is increased, and where the actual slip amount is less than the target slip amount, the torque capacity (transmission torque) of the lockup clutch is increased by a predetermined value and the actual slip amount is reduced.
However, the controller disclosed in JP-A-2005-3193 is limited to causing the actual slip amount to follow the target slip amount during slip control, and the possibility of causing an engine operation point (in particular, engine revolution speed) to follow accurately the optimum point of fuel consumption during the slip control is not considered. As a result, a sufficient room is left for further improving fuel consumption.